1. Field of the Invention
This invention relates to apparatus for high-speed automatic incoming inspection, programming and dynamic testing of programmable read-only memory (PROM) devices. These devices are widely used in the implementation of digital circuits and, therefore, automatic test equipment for these devices would be of particular importance to a manufacturer of large volume digital electronic systems.
2. Description of the Prior Art
Electronic semiconductor devices are manufactured in large quantities and are preliminarily tested by the vendor according to basic specifications before being sent to the user. However, the user usually has a set of specifications that differ from those used in testing by the vendor, or plans to use the devices in a particular application that was not anticipated by the vendor. In almost all cases, therefore, the devices must be subjected to an incoming inspection consisting of electrical testing to the users specifications prior to assembling these devices into electronic circuits Thus, automatic test equipment of greater sophistication must usually be built by device users or by test equipment vendors who sell this test equipment to users. The electronic circuits themselves may either be single transistors, small scale integrated (SSI) devices or medium scale integrated (MSI) devices, and may come in a variety of packages, examples of which would be the TO-5 can and the sixteen pin dual in-line package (DIP). Automatic test equipment for these devices would consist of one or several test stations connected to a battery of signal generating and signal analysing equipment to provide the proper inputs and analyze the appropriate outputs of the device. Where a plurality of test stations are used, each test station may be specialized to a particular function and a performance board engineered to the particular kind of test to be accomplished would be designed into each test station. For instance, at a test station to measure output power, the performance board may be connected to the device through relay contacts, whereas in a timing test station the performance board would necessarily consist of low-capacitance high-speed test apparatus connected to the device under test through wires kept as short as possible.
Each automatic test system also must have transport apparatus that will move the devices from their original container to each test station in turn, ensure electrical contact at each test station, and ultimately sort the devices based upon the results of the tests. Some examples of automatic test equipment in the prior art are Sahakian, U.S. Pat. No. 3,664,499 and Alford, U.S. Pat. No. 3,418,573.
One particular device that has been manufactured in increasingly large quantities recently is the programmable read-only memory (PROM) device. Modern fabrication techniques have brought the cost of these devices down to the point where they are now being used widely in a variety of applications.
One common use of these devices is in the implementation of the control memory of microprogrammable computers. This type of computer relies on control programs permanently stored in a control memory implemented entirely from programmable read-only memory devies. Since a typical device will have a capacity of 256 words by 4 bits and since a typical microprogrammable computer will have a control memory capacity of several thousand words, each between 50 and 100 bits wide, it can be seen that the rapidly enlarging minicomputer industry is a large user of these devices.
Additionally, the cost of PROMs has now been reduced to the point where they can be used as a substitute for random logic. For instance, a matrix can be programmed to logically reduce any number of input variables into any number of output lines. The simplification of circuitry and reductions of cost when replacing random logic with PROM devices can be substantial. In fact, even specialized devices such as arithmetic logic units may be replaced by PROMs at a reduction in cost.
However, as the cost of PROMs decreases and as the use increases, testing of these devices becomes a major criterion in that the cost of programming and testing can easily be a significant fraction of the cost of the device itself. An economical way of programming large volumes of identical PROMs is to generate a mask which can be used during the manufacturing process. However, there are two problems associated with this method. First, if a mistake is discovered in the programming process, a new mask must be generated at considerable expense to correct the error. Furthermore, it would not be commercially feasible to generate a mask for each particular PROM pattern if the PROMs were used for a wide variety of applications in the manufacturing process.
The other common variety of PROM is one in which each matrix point comprises a fuse which may or may not be blown, thus programming that point. Thus, by either blowing or not blowing the 1024 fuses in a 256 word by four bit PROM, the user may program the devices without the necessity and expense of creating masks. However, there are drawbacks to this method as well. The fuse blowing process recommended by the vendor may be a complicated process involving pulses of specified voltage amplitude, current amplitude, compliance voltage, duty cycle, pulse count and rise and fall time. Additionally, the vendor may specify one set of pulse parameters before the fuse is blown and another set of pulse parameters after the fuse is blown to ensure that the fuse will not "heal", which necessitates that the fuse must be monitored during the programming process. Additionally, each word in the device must be programmed differently from the preceding word, and in the general case, each device must be programmed differently from the device preceding it.
A further complication arises since devices manufactured by different vendors vary in their electrical characteristics. For instance, a virgin device from one manufacturer may output all ones whereas the device of another vendor may output all zeros. Likewise, the programming process, the leads where the programming pulses must be applied, the electrial characteristics and the access time all vary not only from vendor to vendor but from model to model manufactured by the same vendor.
For all of the above reasons it is therefore clear that in order to prevent the incoming inspection, programming and dynamic testing of these devices to incur costs that would prevent the economical use of these devices automatic test apparatus for performing all of the above requirements on an economical cost-per-chip basis is required.